BULK GaN AND AlGaN SINGLE CRYSTALS

ABSTRACT

Bulk GaN and AlGaN single crystal boules, preferably fabricated using a modified HVPE process, are provided. The single crystal boules typically have a volume in excess of 4 cubic centimeters with a minimum dimension of approximately 1 centimeter. If desired, the bulk material can be doped during growth, for example to achieve n-, i-, or p-type conductivity.

CROSS-REFERENCES TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 11/114,414 filed Apr. 26, 2005, which is a divisional patentapplication Ser. No. 10/355,426 filed Jan. 31, 2003, now U.S. Pat. No.6,936,357, which in turn is a continuation-in-part of U.S. patentapplication Ser. No. 09/900,833, filed Jul. 6, 2001, now U.S. Pat. No.6,613,143, all of which are incorporated herewith.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor materials and,more particularly, to bulk single crystals.

BACKGROUND OF THE INVENTION

Recent results in the development of GaN-based light-emitting diodes(LEDs) and laser diodes (LDs) operating in the green, blue, andultraviolet spectrum have demonstrated the tremendous scientific andcommercial potential of group III nitride semiconductors (e.g., GaN,AlN, InN, and their alloys). These applications require electricallyconducting substrates (e.g., GaN or AlGaN) so that a vertical devicegeometry can be utilized in which the electrodes are located on the topand bottom surfaces of the device structure. In addition toopto-electronic devices, group III nitride semiconductors can be used ina host of other applications such as communication electronics (e.g.,high power microwave devices). These devices require electricallyinsulating substrates.

In order to achieve the desired device performance (e.g., highefficiency, low leakage current, high device yield, long lifetime, etc.)for devices fabricated from group III nitride semiconductor materials,it is expected that such devices will have to be fabricated on nativeGaN or AlGaN substrates. Native substrates as used herein refers tosubstrates that have been obtained from bulk material, the bulk materialpreferably grown from seeds of the same composition, thus allowing thesubstrates to achieve extremely low defect densities as well as lowresidual stress levels. Unfortunately, due to the lack of bulk GaN andAlGaN substrates, device developers have been forced to attempt variouswork-around solutions.

Initially GaN-based structures were developed on foreign substrates.Some of the substrates that have been used in these attempts aresapphire, silicon carbide, ZnO, LiGaO₂, LiAlO₂, and ScMgAlO₄. Thesedevices suffer from a high defect density including a high density ofdislocations, domains, and grain boundaries. Additionally, these devicessuffer from cracking and bending of the epitaxial structures. Most ofthese problems arise from the lattice and thermal mismatch between theforeign substrates and the GaN-based device structures. As a result,these devices exhibit greatly reduced performance.

In order to attempt to overcome the problems in growing a GaN-baseddevice on a foreign (i.e., non-GaN) substrate, a number of developershave attempted to grow GaN single crystals suitable for use withmicroelectronic device structures. For example, U.S. Pat. No. 5,679,152discloses a technique for growing a GaN layer on a sacrificial substrateand then etching away the substrate. As disclosed, the substrate isetched away in situ while the substrate/GaN layer is at or near thegrowth temperature. The GaN layer may be deposited directly on thesacrificial substrate or deposited on an intermediate layer that hasbeen deposited on the substrate.

Alternately, U.S. Pat. No. 5,770,887 discloses a repetitive growthtechnique utilizing alternating epitaxially grown layers of a buffermaterial and GaN single crystal. Removing the buffer layers, for exampleby etching, produces a series of GaN single crystal wafers. The patentdiscloses using sapphire as the initial seed crystal and a material suchas BeO, MgO, CaO, ZnO, SrO, CdO, BaO or HgO as the buffer layermaterial.

Another technique for growing GaN substrates is disclosed in U.S. Pat.No. 6,146,457. As disclosed, a thick layer of GaN is epitaxiallydeposited on a thin, disposable substrate using a CVD process. In thepreferred embodiment, the substrate (e.g., sapphire) has a thickness of20-100 microns while the GaN epitaxial layer has a thickness of 50-300microns. Accordingly, upon material cooling, the strain arising from thethermal mismatch of the material is relieved by cracking the disposablesubstrate rather than the newly deposited epitaxial layer. As noted bythe inventors, however, the disclosed process solves the problemsassociated with thermal mismatch, not lattice mismatch.

U.S. Pat. No. 6,177,292 discloses a method for growing a GaN groupmaterial on an oxide substrate without cracking. After the growth of theGaN group material, a portion of the oxide substrate is removed bymechanical polishing. Another GaN layer is then grown on the initial GaNlayer to provide further support prior to the complete removal of theremaining oxide substrate. As a result of this process, a stand-aloneGaN substrate is formed that can be used as a substrate for the growthof a micro-electronic device.

U.S. Pat. No. 6,218,280 discloses a method and apparatus for depositingIII-V compounds that can alternate between MOVPE and HVPE processes,thus not requiring that the substrate be cooled and transported betweenthe reactor apparatus during device growth. As disclosed, the MOVPEprocess is used to grow a thin III-V nitride compound semiconductorlayer (e.g., a GaN layer) on an oxide substrate (e.g., LiGaO₂, LiAlO₂,MgAlScO₄, Al₂MgO₄, and LiNdO₂). The HVPE process is then used to growthe device structure on the GaN layer.

The main problems associated with growing true bulk GaN or AlGaNcrystals relate to fundamental properties of the materials. For example,sublimation growth of GaN is limited by the incongruent decomposition ofGaN that becomes noticeable at temperatures from 800° C. to 1100° C. InU.S. Pat. No. 6,136,093, a technique is disclosed for growing GaN on aGaN seed. As disclosed, Ga is heated to or above the evaporationtemperature of Ga and the seed crystal is heated to a temperature higherthan that of the Ga. The Ga vapor then reacts with a nitrogen containinggas to form GaN which is deposited on the seed crystal.

Another method to grow bulk GaN crystals is to grow them from the liquidphase. The main problem associated with liquid phase growth of GaN isthe extremely low solubility of nitrogen in melts in general, and in Gamelts in particular. The low solubility limits the GaN growth rate andresults in small volume GaN crystals, generally less than 0.2 cubiccentimeters.

In order to overcome the low solubility of nitrogen in the Ga melts,typically growth temperatures between 1500° C. and 1600° C. are usedwith a nitrogen gas pressure in the range of 10 to 20 kilobars. Even atthese high pressures and temperatures, nitrogen solubility in the Gamelt is very low. As a result, only growth rates of approximately 0.01to 0.05 millimeters per hour can be obtained. Lateral growth rate, i.e.,the growth rate perpendicular to the [0001] crystallographic direction,is typically about 1 millimeter per day. In addition to the low growthrates, due to the high temperatures and pressures it is difficult todevelop production techniques using this process.

Accordingly, although III-V nitride compound semiconductor materialsoffer tremendous potential for a variety of micro-electronic devicesranging from opto-electronic devices to high-power, high-frequencydevices, the performance of such devices is limited by the lack ofsuitable substrates. The present invention provides suitable substrates.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for growing bulkGaN and AlGaN single crystal boules, preferably using a modified HVPEprocess, as well as the resultant material. The single crystal boulesfabricated in accordance with the present invention typically have avolume in excess of 4 cubic centimeters with a minimum dimension (i.e.,x, y, or z dimension) of approximately 1 centimeter.

According to one aspect of the invention, an extended Ga source islocated within a reactor such that a portion of the Ga source ismaintained at a relatively high temperature while most of the Ga sourceis maintained at a temperature close to, and just above, the meltingtemperature of Ga. In at least one embodiment of the invention, in orderto obtain the desired temperature spread in the Ga source, a portion ofthe source tube extends outside of the reactor. Assuming the use of amodified HVPE process for the growth of the single crystal boule,preferably the high temperature is in the range of 450° C. to 850° C.,and more preferably at a temperature of approximately 650° C., while thelow temperature is less than 100° C. and preferably in the range of 30°C. to 40° C. As a result of this source configuration, the amount of Gaundergoing a reaction is controllable and limited, thus allowingextended growth cycles to be realized without experiencing degradationin the as-grown material.

In another aspect of the invention, an extended Al source, preferablycomprised of one or more individual Al sources, is included in thereactor, thereby allowing the growth of AlGaN boules. Although a singleAl source can be used, in order to accomplish the desired extendedgrowth cycles, thereby allowing the growth of large single crystalboules, multiple Al sources are used. If multiple Al sources are used,they are sequentially activated. Accordingly, as one Al source begins todegrade and/or become depleted, another Al source is activated and thefirst source is deactivated. The cycling of Al sources continues as longas necessary in order to complete the growth process.

In at least one embodiment of the invention, the reactor is at leastpartially surrounded by a multi-temperature zone furnace. The reactorincludes at least one, and preferably two, growth zones. One of thegrowth zones is maintained at a high temperature, preferably within therange of 1,000° C. to 1,100° C. This growth zone is preferably used toinitiate crystalline growth. Although the high temperature of this zoneallows high quality crystal growth, the growth rate at this temperatureis relatively low. Accordingly a second growth zone, preferably held toa temperature within the range of 850° C. to 1,000° C., is used aftercrystal growth is initiated. The crystal growth rate at this temperatureis relatively high.

In at least one embodiment of the invention, the reactor includes anextended Ga source within one source tube, the Ga source tube connectedto a source of a halide gas (e.g., HCl) and to a source of an inert gas(e.g., Ar). During the growth cycle, the halide gas is introduced intothe Ga source tube where it primarily reacts with the Ga held at a hightemperature. As a result of the reaction, a halide metal compound isformed which is delivered to the growth zone by the inert gas.Simultaneously, ammonia gas is delivered to the growth zone. As a resultof the reaction of the halide metal compound and the ammonia gases, GaNis deposited on one or more seed substrates within the growth zone. Bysimultaneously supplying aluminum trichloride to the growth zone, forexample by reacting an Al source with HCl, AlGaN is grown on the seedcrystals within the growth zone. Additionally, by supplying a suitabledopant to the growth zone during the growth cycle, GaN or AlGaN of n-,i-, or p-type conductivity can be grown.

In at least one embodiment of the invention, in order to achievesuperior quality in a single crystal boule of GaN or AlGaN, a repetitivegrowth cycle is used. During the first growth cycle, a single crystal ofthe desired material (e.g., GaN or AlGaN) is grown on a seed substrateof a different chemical composition. Suitable seed crystals includesapphire, silicon carbide, and gallium arsenide (GaAs). After completionof a relatively lengthy growth cycle, preferably of sufficient durationto grow a single crystal several millimeters thick, the substrate isremoved. The remaining crystal is then subjected to suitable surfacepreparatory steps. This crystal is then used as the seed crystal foranother extended growth cycle, preferably of sufficient duration to growa single crystal boule of approximately 1 centimeter in thickness.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a horizontal furnace as used withthe invention;

FIG. 2 is an illustration of one embodiment of a boat suitable for usewith the furnace shown in FIG. 1;

FIG. 3 is an illustration of an individual source tube and a means ofvarying the source contained within the tube relative to the reactor;

FIG. 4 is a block diagram outlining the preferred method of fabricatingbulk GaN according to the invention;

FIG. 5 is a schematic illustration of an alternate embodiment for use ingrowing AlGaN;

FIG. 6 is a block diagram outlining the preferred method of fabricatingbulk AlGaN according to the invention;

FIG. 7 is a schematic illustration of an alternate embodiment for use ingrowing doped material; and

FIG. 8 outlines a process used in at least one embodiment of theinvention to grow material with a matching seed crystal.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The present invention provides a method and apparatus for growing singlecrystal bulk gallium nitride (GaN) or aluminum gallium nitride (AlGaN),preferably using a modified hydrid vapor phase epitaxial (HVPE)approach, as well as the resultant material. The single crystal boulesfabricated in accordance with the present invention preferably have avolume in excess of 4 cubic centimeters with a minimum dimension (i.e.,x, y, or z dimension) of approximately 1 centimeter; more preferably,the single crystal boules fabricated in accordance with the presentinvention have a volume in excess of 10 cubic centimeters with a minimumdimension (i.e., x, y, or z dimension) of approximately 1 centimeter;and yet still more preferably, the single crystal boules fabricated inaccordance with the present invention have a volume in excess of 20cubic centimeters with a minimum dimension (i.e., x, y, or z dimension)of approximately 1 centimeter.

Single crystal boules fabricated in accordance with the inventionpreferably have a dislocation density of less than 100,000 cm⁻², morepreferably less than 10,000 cm⁻², and still more preferably less than1,000 cm⁻².

FIG. 1 is a schematic illustration of a horizontal furnace as used withthe invention. It is understood that the invention is not limited tothis particular furnace configuration as other configurations (e.g.,vertical furnaces) that offer the required control over the temperature,temperature zone or zones, gas flow, source and substrate location,source configuration, etc., can also be used. The furnace configurationillustrated in FIG. 1 is preferred for the growth of undoped GaN as iteasily accommodates the desired gallium source of the invention.

Furnace 100 is comprised of multiple temperature zones, preferablyobtained through the use of multiple heaters 101, each of which at leastpartially surrounds reactor tube 103. In the preferred embodiment, a sixzone configuration is used in which heaters 101 are resistive heaters.It is understood that although reactor tube 103 preferably has acylindrical cross-section, other configurations can be used such as a‘tube’ with a rectangular cross-section. Within reactor tube 103 are oneor more source tubes 105. As noted with respect to reactor tube 103,although source tubes 105 preferably have a cylindrical cross-section,the invention is not limited to cylindrical source tubes.

In order to grow undoped bulk GaN, a single source tubes 105 isrequired. Within source tubes 105 is a source boat 107. As used herein,the term “boat” simply refers to a means of holding the source material.For example, boat 107 may be comprised of a portion of a boat 201 with apair of end portions 203 as illustrated in FIG. 2. Alternately, thesource material can be held within source tubes 105 without the use of aseparate boat 107. Alternate boat configurations are clearly envisionedby the inventors.

As described in detail below, in the preferred embodiment of theinvention the desired growth temperature depends upon the stage ofcrystal growth (e.g., crystal nucleation versus high growth rate). Thetemperature of a source in general, and the temperature of a specificportion of the gallium source in particular, are preferably controlledby varying the heat applied by specific heaters 101. Additionally, inthe preferred embodiment of the invention in which multiple source typesare used, the location of a particular source (e.g., an impurity source)relative to reactor tube 103 can be controllably varied, typically byaltering the position of the source. For example, as illustrated in FIG.3, a source tube 301 typically includes a boat 303, a source 305 withinboat 303, and a gas inlet 307. A control rod 309 coupled to boat 303 canbe used to alter the position of the boat, and thus the source, withinthe reactor. Control rod 309 can be manually manipulated, as providedfor in the illustrated configuration, or coupled to a roboticpositioning system (not shown).

In the preferred embodiment, coupled to each source tube are one or moresources of gas 109-111. The rate of gas flow through a particular sourcetube is controlled via valves 113-115, either manually or by anautomatic processing system.

A substrate 117 is located on a pedestal 119 within the growth zone ofreactor 103. Although typically multiple substrates 117 are manuallyloaded into the reactor for co-processing, a single substrate can beprocessed with the invention. Additionally, substrates 117 can beautomatically positioned within the furnace for automated productionruns. In order to vary the temperature of the growth zone, and thussubstrate or substrates 117, either the position of the substratesrelative to reactor 103 are changed or the amount of heat applied byheaters 101 proximate to the growth zone is varied.

FIGS. 1 and 4 illustrate a specific reactor 100 and the steps used togrow bulk GaN, respectively. Although reactor 100 is a hot-wall,horizontal reactor and the process is carried out in an inert gas flowat atmospheric pressure, as previously noted other reactorconfigurations can be used to perform the modified HVPE process of theinvention. Preferably source tubes 105 and source boat 107 are comprisedof quartz. Other materials can be used for boat 107, however, such assapphire or silicon carbide. Within boat 107, or simply within tubes 105if no separate boat is used, is a Ga metal source 121.

In order to achieve extended GaN growth, as required to grow bulk GaN,the inventors have found that an extended source of Ga must be used andthat the extended source must be maintained at more than onetemperature. Specifically, Ga metal 121 is positioned relative toreactor 103 such that a large quantity of source 121 (i.e., preferablygreater than 90 percent of source 121) is maintained at a relatively lowtemperature, preferably less than 100° C. and more than the meltingtemperature of Ga (i.e., 29.78° C.), and more preferably within thetemperature range of 30° C. to 40° C. Due to the low temperature, thisportion of Ga source 121 has limited reaction with the halide reactivegas coupled to and flowing through source tube 105. If desired, as inthe preferred embodiment, a portion of source tubes 105 and Ga source121 are maintained outside of the reactor volume as illustrated inFIG. 1. Alternately, the lower temperature of this portion of source 121is achieved through control of heaters 101 adjacent to the lowertemperature portion of the source.

At the high temperature end of source tube 105, the temperature of Gasource 121 is held at a relatively high temperature, typically between450° C. and 850° C. and preferably at a temperature of approximately650° C. During crystal growth, a constant source of Ga is maintained dueto the flow of Ga from the low temperature portion of tubes 105 to thehigher temperature portion of tube 105. Accordingly, by providing alarge Ga source, the present invention allows the growth of bulk GaNwhile limiting the amount of the source that reacts with the halidereactive gas. It is understood that although the preferred embodiment ofthe invention utilizes a modified HVPE process in conjunction with thelarge Ga source described above, the source can be used with other bulkgrowth techniques (e.g., sublimation techniques).

In order to grow bulk GaN according to the preferred embodiment of theinvention, a source of halide gas 109, preferably HCl, is coupled tosource tubes 105 along with a source of inert gas 110, preferably Ar. Asource of ammonia gas 111 is also coupled to reactor 103. In order togrow bulk GaN, preferably seed crystals 117 are comprised of GaN, thusproviding a lattice and coefficient of thermal expansion match betweenthe seed and the material to be grown. As a result of using GaN seedcrystals, improved quality in the as-grown material is achieved.Alternately, seed crystals 117 can be of silicon carbide (SiC),sapphire, gallium arsenide (GaAs), or other material. Seed crystalpedestal 119 is preferably fabricated from quartz, although othermaterials such as silicon carbide or graphite can also be used.

Initially reactor 103 is flushed and filled with an inert gas,preferably Ar, from gas source 110, (step 401). The inert gas can enterthe reactor through source tube 105, thereby flushing the source tube,through a separate entry line (not shown), or both. The flow of inertgas is controlled by metering valve 114 and is typically in the range of1 to 25 liters per minute. Substrates (or substrate) 117 are then heatedto the desired growth temperature (step 403). In one embodiment of theinvention the growth zone, and thus the substrates within the growthzone, are heated to a temperature within the range of 1,000° C. and1,100° C. This temperature achieves a higher quality material in theas-grown crystal, but yields relatively slow growth rate. In analternate embodiment, the growth zone is maintained at a temperaturewithin the range of 850° C. and 1,000° C. Although this temperature iscapable of fast crystal growth, the resulting crystal is of lowerquality. In the preferred embodiment of the invention, the methodologyof which is illustrated in FIG. 4, the growth zone and thus thesubstrates (or substrate) within the growth zone are initially heated toa high temperature within the range of 1,000° C. and 1,100° C., thusinitiating high quality crystal growth. Once crystal growth has beeninitiated, the source temperature is lowered and maintained at atemperature within the range of 850° C. and 1,000° C., thus allowingrapid crystal growth to be achieved. Preferably the period of highquality crystal growth is at least 10 minutes and the period of rapidcrystal growth is at least 12 hours. More preferably the period of highquality crystal growth is at least 30 minutes and the period of rapidcrystal growth is at least 24 hours.

Preferably prior to initiating crystal growth, the surfaces ofsubstrates 117 are etched to remove residual surface contamination, forexample by using gaseous HCl from supply 109. The Ga source material 121is initially heated to a temperature sufficient to cause the entiresource to melt (step 405). As previously noted, the melting temperatureof Ga is 29.78° C. and source 121 is preferably heated to a temperaturewithin the range of 30° C. to 40° C. A portion of source tubes 105closest to substrates 117, and thus the portion of source material 121closest to substrates 117, is heated to a relatively high temperature(step 407), typically between 450° C. and 850° C. and preferably at atemperature of approximately 650° C.

After the source material is heated a halide reactive gas, preferablyHCl, is introduced into source tubes 105 (step 409). As a result of thereaction between HCl and Ga, gallium chloride is formed which istransported to the reactor's growth zone by the flow of the inert (e.g.,Ar) gas (step 411). Simultaneously, ammonia gas (NH₃) from source 111 isdelivered to the growth zone (step 413). The NH₃ gas and the galliumchloride gas react (step 415) to form GaN on the surface of seedsubstrates 117 (step 417). The initial growth rate of the GaN is in therange of 0.05 to 1 micron per minute. After a high quality GaN layer ofsufficient thickness has been grown, typically on the order of 20microns and preferably on the order of 50 microns, the temperature ofthe growth zone is lowered (step 419) to a temperature within the rangeof 850° C. and 1,000° C., thereby allowing GaN to be grown at anaccelerated rate (i.e., in the range of 5 to 500 microns per hour).After the desired boule thickness has been achieved, the flow of HCl andNH₃ gas is stopped and substrates 117 are cooled in the flowing inertgas (step 421).

After the intended boule thickness has been achieved, the boule(s) isprocessed (step 423) as desired. Typically boule processing includesgrinding the boule into a specific shape, preferably a cylindricalshape, and then cutting the processed boule into individual wafers. Thewafers may be oriented as desired, for example, in at least oneapplication the wafers preferably have a crystallographic surfaceorientation of (0001). Alternately, the (0001) surface can be tiltedbetween 0 and 90 degrees to obtain a specifically desiredcrystallographic direction. Typically at least one surface of each waferis polished, preferably to a surface roughness of less than 3nanometers, with individual wafers preferably having a dislocationdensity in the range of 10 to 100,000 cm⁻², more preferably in the rangeof 10 to 10,000 cm⁻², and still more preferably in the range of 10 to1,000 cm⁻². Preferably the processed material has a full width at halfmaximum (FWHM) of the x-ray rocking curve measured in .omega.-scanninggeometry ranging from 60 to 360 arc seconds, more preferably an FWHM ofless than 360 arc seconds, and still more preferably an FWHM of lessthan 90 arc seconds.

FIGS. 5 and 6 illustrate another embodiment of the invention that can beused to grow AlGaN boules. Reactor 500 is substantially the same asreactor 100 except for the inclusion of an aluminum (Al) source. Also inthis embodiment Ga source tube 105 is shown to be completely within thereactor. As the Al source tends to degrade over time due to the reactionbetween the Al and the source tube/boat materials, in the preferredembodiment of the invention, reactor 500 includes multiple Al sources.As shown, reactor 500 includes three Al source tubes 501, although it isunderstood that fewer or greater numbers of Al source tubes can beincluded, depending upon the quantity of AlGaN to be grown. Within eachAl source tube 501 is a source boat 503 containing a quantity of Almetal 505. Preferably each source boat 503 is fabricated from sapphireor silicon carbide. Additionally, as discussed with reference to FIG. 3,the position of each source boat 503 within the reactor can be alteredusing either a manual or automatic control rod 507.

As previously noted, preferably the seed crystal is of the same materialas the crystal to be grown. Therefore in order to grow bulk AlGaN,preferably seed crystal 609 is fabricated of AlGaN. Alternately, seedcrystal 609 can be of GaN, SiC, sapphire, GaAs, or other material.

The methodology to grow AlGaN is very similar to that outlined in FIG. 4for GaN growth. In this embodiment, during source heating one of the Alsources 505 is heated to a temperature of preferably between 700° C. and850° C. (step 601), the selected Al source being appropriatelypositioned within the reactor to achieve the desired temperature. Onceall of the materials have achieved the desired growth temperature,halide gas (e.g., HCl) is introduced into Ga source tubes 105 and theselected Al source tube (step 603). As a result, gallium chloride andaluminum trichloride are formed (step 605). Both the gallium chlorideand aluminum chloride are transported to the growth zone using an inertgas (e.g., Ar) (step 607). NH₃ gas 111 is simultaneously introduced intothe growth zone with the source materials (step 609) resulting in areaction by the three gases to form AlGaN (step 611). As in the priorembodiment, preferably the growth zone is initially held at a highertemperature in order to initiate the growth of high quality material.Once a sufficiently large layer is formed, preferably on the order of 50microns thick, the temperature of the growth zone is lowered (step 419)to a temperature within the range of 850° C. and 1,000° C. in order toachieve accelerated growth. Prior to exhaustion or excessive degradationof the initially selected Al source, a second Al source 503 is heated toa temperature within the preferred range of 700° C. and 850° C. (step613). Once the second Al source is heated, halide gas (e.g., HCl) isintroduced into the second Al source tube (step 615) and the resultantaluminum trichloride is transported to the growth zone (step 617). Theflow of halide and inert gas through the initially selected Al sourcetube is then stopped and the first Al source is withdrawn from the hightemperature zone (step 619). The process of introducing new Al sourcescontinues as long as necessary to grow the desired AlGaN boule. Afterthe desired boule thickness has been achieved, the flow of HCl and NH₃gas is stopped, substrates 117 are cooled in the flowing inert gas (step421) and the boule(s) is processed as desired and as previouslydescribed (step 423).

In addition to undoped GaN and AlGaN bulk material, the invention canalso be used to grow doped GaN and AlGaN bulk material. Typical dopantsinclude oxygen (O), silicon (Si), germanium (Ge), tin (Sn), magnesium(Mg), zinc (Zn), iron (Fe), chromium (Cr), indium (In), carbon (C) andboron (B). Control of the type and concentration of dopant(s) allows theconductivity of the resultant material to be controlled.

FIG. 7 illustrates another embodiment of the invention that allows theaddition of dopants during crystal growth. The embodiment shown includesGa source tube 105, two Al source tubes 501, and two dopant source tubes701. It is understood that the number of source tubes is based on thenumber of constituents required for the desired crystal.

To grow p-type GaN or AlGaN, a suitable dopant (i.e., acceptor) isplaced within one or more boats 703 within one or more dopant sourcetubes 701, thus allowing the desired dopants to be added to the crystalduring growth. Preferably either magnesium (Mg) or a combination of Mgand zinc (Zn) is used. If multiple dopants are used, for example both Mgand Zn, the dopants may be in the form of an alloy, and thus be locatedwithin a single boat, or they may be in the form of individualmaterials, and thus preferably located within separate boats. To growinsulating (i.e., i-type) GaN or AlGaN, preferably Zn is used as thedopant. Although undoped GaN and AlGaN exhibit low n-type conductivity,controllable n-type conductivity can be achieved by doping the growingcrystal with donors. Preferred donors include silicon (Si), germanium(Ge), tin (Sn), and oxygen (O).

A detailed discussion of GaN and AlGaN doping is provided in co-pendingU.S. patent application Ser. No. 09/861,011, pages 7-14, the teachingsof which are hereby incorporated by reference for any and all purposes.In the preferred embodiment of the invention, dopant source boats 703are formed of non-reactive materials (e.g., sapphire), extremely puresource materials are used (e.g., 99.999 to 99.9999 purity Mg), and thesource materials are etched prior to initiating the growth process toinsure minimal surface contamination. Although the temperature for aparticular dopant source depends upon the selected material, typicallythe temperature is within the range of 250° C. to 1050° C. If a Mgdopant is used, preferably the temperature is within the range of 450°C. to 700° C., more preferably within the range of 550° C. to 650° C.,and still more preferably at a temperature of approximately 615° C. Thedopant source or sources are heated simultaneously with the substrateand the Ga or the Ga and Al sources. The dopants are delivered to thegrowth zone via inert gas (e.g., Ar) flow. The flow rate depends uponthe conductivity to be achieved in the growing crystal. For example, forgrowth of p-type GaN or AlGaN, the flow rate for a Mg dopant istypically between 1,000 and 4,000 standard cubic centimeters per minute,and preferably between 2,000 and 3,500 standard cubic centimeters perminute.

As previously described, the level of doping controls the conductivityof the grown material. In order to achieve p-type material, it isnecessary for the acceptor concentration (N_(a)) to be greater than thedonor concentration (N_(d)). The inventors have found that in order toachieve the desired N_(a)/N_(d) ratio and grow p-type GaN or AlGaN, theconcentration of the acceptor impurity must be in the range of 10¹⁸ to10²¹ atoms per cubic centimeter, and more preferably in the range of10¹⁹ to 10²⁰ atoms per cubic centimeter. For an i-type layer, the dopinglevel must be decreased, typically such that the dopant concentrationdoes not exceed 10¹⁹ atoms per cubic centimeter.

As previously noted, improved crystal quality in the as-grown materialis achieved when the seed crystal and the material to be grown are ofthe same chemical composition so that there is no crystal lattice orcoefficient of thermal expansion mismatch. Accordingly, FIG. 8 outlinesa process used in at least one embodiment of the invention in whichmaterial is grown using a matching seed crystal.

In the illustrated embodiment of the invention, initially material(e.g., doped or undoped GaN or AlGaN) is grown from a seed crystal ofdifferent chemical composition using the techniques described in detailabove (step 801). As previously noted, the seed crystal can be ofsapphire, silicon carbide, GaAs, or other material. After the bulkmaterial is formed, a portion of the grown crystal is removed from thebulk for use as a new seed crystal (step 803). For example, new seedcrystals can be obtained by cutting off a portion of the as-grown bulk(step 805) and subjecting the surfaces of the cut-off portion tosuitable surface preparatory steps (step 807). Alternately, prior tocutting up the as-grown bulk material, the initial seed crystal can beremoved (step 809), for example using an etching technique. Once a newseed crystal is prepared, the bulk growth process of the presentinvention is used to grow a second crystal (step 811). However, as aconsequence of the ability to grow bulk materials according to theinvention, the second growth cycle is able to utilize a seed crystal ofthe same composition as the material to be grown, thus yielding asuperior quality material.

SPECIFIC EMBODIMENTS

Embodiment 1

According to this embodiment of the invention, the modified HVPE processdescribed above was used to grow thick GaN layers on SiC substrates.Suitable GaN substrates were then fabricated and used in conjunctionwith the modified HVPE process of the invention to grow a GaN singlecrystal boule. The second GaN boule was cut into wafers suitable fordevice applications.

In this embodiment, multiple SiC substrates of a 6H polytype were loadedinto the growth zone of a reactor similar to that shown in FIG. 1. Thesubstrates were placed on a quartz sample holder with the (0001) Sion-axis surface positioned for GaN deposition. One kilogram of Ga metalwas positioned in the source boat within the Ga source tube. Afterpurging the reactor with Ar gas to remove air, the growth zone and theGa source zone were heated to 1100° C. and 650° C., respectively. Themajority of the Ga source, however, was maintained at a temperature ofless than 100° C., typically in the range of 30° C. to 40° C. To preparethe substrates for GaN deposition, HCl gas was introduced into thegrowth zone to etch the SiC substrates. The HCl gas was then introducedinto the Ga source zone, thereby forming gallium chloride that wastransported into the growth zone by the Ar carrier gas. Simultaneously,NH₃ gas was introduced into the growth zone, the NH₃ gas providing asource of nitrogen. As a result of the reaction between the galliumchloride and the NH₃ gases, a GaN layer was grown on the SiC surface.The NH₃ and gallium chloride gases were expelled from the reactor by theflow of the Ar gas. After allowing the growth process to continue for aperiod of 24 hours, the flow of HCl and NH₃ gases was stopped and thefurnace was slowly cooled down to room temperature with Ar gas flowingthrough all of the gas channels. The reactor was then opened to the airand the sample holder was removed. As a result of this growth process,GaN layers ranging from 0.3 to 2 millimeters were grown on the SiCsubstrates. The range of GaN thicknesses were the result of thedistribution of GaN growth rates within the growth zone.

To prepare GaN seed substrates, the SiC substrates were removed from thegrown GaN material by chemically etching the material in molten KOH. Theetching was carried out in a nickel crucible at a temperature within therange of 450° C. to 650° C. Prior to beginning the etching process, themolten KOH was maintained at the etching temperature for several hoursto remove the moisture from the melt and the crucible. Once thesubstrates were placed within the molten KOH, only a few hours wererequired to etch away most of the SiC substrates from the grown GaN.This process for substrate removal is favored over either mechanical orlaser induced substrate removal. The remaining SiC substrate was removedby reactive ion etching in a Si₃F/Ar gas mixture. For some of theas-grown material, polycrystalline material was noted in the peripheralregions, this material being subsequently removed by grinding.Additionally, in some instances the surface of the as-grown materialrequired mechanical polishing to smooth the surface. In these instances,after the polishing was completed, reactive ion etching or chemicaletching was used to remove the thin surface layer damaged duringpolishing. As a result of this procedure, the desired GaN seeds wereobtained. The high quality of the resultant material was verified by thex-ray rocking .omega.-scan curves (e.g., 300 arc sec for the FWHM forthe (0002) GaN reflection). X-ray diffraction measurements showed thatthe as-grown material was 2H—GaN.

The inventors have found that SiC substrates are preferable oversapphire substrates during the initial growth process as the resultantmaterial has a defined polarity. Specifically, the resultant materialhas a mixture of gallium (Ga) polarity and nitrogen (N) polarity. Theside of the as-grown material adjacent to the SiC substrates has an Npolarity while the opposite, outermost layer of the material has a Gapolarity.

Prior to growing a GaN boule utilizing the process of the invention, insome instances the inventors found that it was beneficial to grow a thinGaN layer, e.g., typically in the range of 10 to 100 microns thick, onone or both sides of the GaN substrates grown above. The additionalmaterial improved the mechanical strength of the substrates and, ingeneral, prepared the GaN surface for bulk growth. Prior to bulk growth,the GaN seed substrates were approximately 1 millimeter thick andapproximately 6 centimeters in diameter.

The growth of the GaN boule used the same reactor as that used to growthe GaN seed substrates. The substrates were positioned within thereactor such that the new material would be grown on the (0001) Gaon-axis face. The inventors have found that the Ga face is preferredover the N face as the resulting boule has better crystal properties andlower dislocation density. It should be noted that the (0001) surfacecan be tilted to a specific crystallographic direction (e.g., [11-20]and that the tilt angle may be varied between 0.5 and 90 degrees. In thepresent embodiment, the tilt angle was zero.

In addition to loading the seed substrates into the growth zone of thereactor, two kilograms of Ga was loaded into the source boat within theGa source tube. After purging the reactor with Ar gas, the growth zoneand the Ga source zone were heated to 1050° C. and 650° C.,respectively. As previously described, only a small portion of the Gasource was brought up to the high source temperature noted above (i.e.,650° C.). Most of the Ga source was maintained at a temperature close toroom temperature, typically in the range of 30° C. and 40° C. Prior toinitiating GaN growth, a mixture of NH₃ and HCl gas was introduced inthe growth zone to refresh the GaN seed surface. As in the growth of theseed crystal previously described, HCl was introduced into the Ga sourcezone to form gallium chloride that was then transported to the growthzone with Ar gas. At the same time, NH₃ gas used as a source of nitrogenwas introduced into the growth zone. The GaN was formed by the reactionbetween the gallium chloride and the NH₃ gases.

After approximately 30 minutes of GaN growth, the GaN substrate wasmoved into a second growth zone maintained at a temperature ofapproximately 980° C., thereby achieving accelerated growth rates aspreviously disclosed. This process was allowed to continue forapproximately 80 hours. After that, HCl flow through the Ga source tubeand NH₃ flow though the growth zone were stopped. The furnace was slowcooled down to room temperature with Ar flowing through all gaschannels. The reactor was then opened to the air and the sample holderwas removed from the reactor. The resultant boule had a diameter ofapproximately 6 centimeters and a thickness of approximately 1centimeter. The crystal had a single crystal 2H polytype structure asshown by x-ray diffraction measurements.

After growth, the boule was machined to a perfect cylindrical shape witha 5.08 centimeter diameter (i.e., 2 inch diameter), thereby removingdefective peripheral areas. One side of the boule was ground to indicatethe (11-20) face. Then the boule was sliced into 19 wafers using ahorizontal diamond wire saw with an approximately 200 micron diamondwire. Before slicing, the boule was oriented using an x-ray technique inorder to slice the wafers with the (0001) oriented surface. The slicingrate was about 1 millimeter per minute. The wire was rocked around theboule during the slicing. Thickness of the wafers was varied from 150microns to 500 microns. Wafer thickness uniformity was better than 5percent.

After slicing, the wafers were polished using diamond abrasivesuspensions. Some wafers were polished only on the Ga face, some waferswere polished only on the N face, and some wafers were polished on bothsides. The final surface treatment was performed using a reactive ionetching technique and/or a chemical etching technique to remove thesurface layer damaged by the mechanical treatment. The surface of thewafers had a single crystal structure as shown by high energy electrondiffraction techniques. The surface of the finished GaN wafers had amean square roughness, RMS, of 2 nanometers or less as determined byatomic force microscopy utilizing a viewing area of 5 by 5 microns. Thedefect density was measured using wet chemical etching in hot acid. Fordifferent wafers, etch pit density ranged from 10 to 1000 per squarecentimeter. Some GaN wafers were subjected to heat treatment in an argonatmosphere in a temperature range from 450° C. to 1020° C. in order toreduce residual stress. Raman scattering measurements showed that suchheat treatment reduced stress from 20 to 50 percent.

In order to compare the performance of devices fabricated using the GaNsubstrates fabricated above to those fabricated on SiC and sapphire, GaNhomoepitaxial layers and pn diode multi-layer structures were grown.Device structures included AlGaN/GaN structures. Prior to devicefabrication, surface contamination of the growth surface of the GaNwafers was removed in a side growth reactor with a NH₃—HCl gas mixture.The thickness of individual layers varied from 0.002 micron to 200microns, depending upon device structure. For example, high frequencydevice structures (e.g., heterojunction field effect transistors) hadlayers ranging 0.002 to 5 microns. For high power rectifying diodes,layers ranged from 1 to 200 microns. In order to obtain p-type layers, aMg impurity was used while n-type doping was obtained using a Siimpurity. The fabricated device structures were fabricated employingcontact metallization, photolithography and mesa insulation.

The structures fabricated on the GaN wafers were studied using opticaland electron microscopy, secondary ion mass spectrometry,capacitance-voltage and current-voltage methods. The devices showedsuperior characteristics compared with devices fabricated on SiC andsapphire substrates. Additionally, it was shown that wafer surfacecleaning procedure in the reactor reduced defect density, includingdislocation and crack density, in the grown epitaxial layers.

Embodiment 2

In this embodiment, a GaN seed was first fabricated as described inEmbodiment 1. The 5.08 centimeter diameter (i.e., 2 inch diameter)prepared GaN seed substrates were then placed within a stainless steel,resistively heated furnace and a GaN single crystal boule was grownusing a sublimation technique. GaN powder, located within a graphiteboat, was used as the Ga vapor source while NH₃ gas was used as thenitrogen source. The GaN seed was kept at a temperature of 1100° C.during the growth. The GaN source was located below the seed at atemperature higher than the seed temperature. The growth was performedat a reduced pressure.

The growth rate using the above-described sublimation technique wasapproximately 0.5 millimeters per hour. After a growth cycle of 24hours, a 12 millimeter thick boule was grown with a maximum boulediameter of 54 millimeters. The boule was divided into 30 wafers using adiamond wire saw and the slicing and processing procedures described inEmbodiment 1. X-ray characterization was used to show that the GaNwafers were single crystals.

Embodiment 3

In this embodiment, bulk GaN material was grown in an inert gas flow atatmospheric pressure utilizing the hot-wall, horizontal reactordescribed in Embodiment 1. Six 5.08 centimeter diameter (i.e., 2 inchdiameter) silicon carbide substrates of a 6H polytype, were placed on aquartz pedestal and loaded into a growth zone of the quartz reactor. Thesubstrates were located such that the (0001) Si on-axis surfaces werepositioned for GaN deposition. Approximately 0.9 kilograms of Ga (7N)was located within a quartz boat in the Ga source zone of the reactor.This channel was used for delivery of gallium chloride to the growthzone of the reactor. A second quartz tube was used for ammonia (NH₃)delivery to the growth zone. A third separate quartz tube was used forHCl gas delivery to the growth zone.

The reactor was filled with Ar gas, the Ar gas flow through the reactorbeing in the range of 1 to 25 liters per minute. The substrates werethen heated in Ar flow to a temperature of 1050° C. and the hot portionof the metal Ga source was heated to a temperature in the range of 350°C. to 800° C. The lower temperature portion of the Ga source wasmaintained at a temperature within the range of 30° C. to 40° C. HCl gaswas introduced into the growth zone through the HCl channel. As aresult, the SiC seed substrates were etched at Ar—HCl ambient beforeinitiating the growth procedure.

To begin the growth process, HCl gas was introduced into the Ga sourcezone, creating gallium chloride that was delivered to the growth zone byAr gas flow. Simultaneously, NH₃ was introduced into the growth zone. Asa result of the reaction between the gallium chloride gas and theammonia gas, a single crystal epitaxial GaN layer was grown on thesubstrates. The substrate temperature during the growth process was heldconstant at 1020° C. After a growth period of 20 hours, the flow of HCland NH₃ were stopped and the samples were cooled in flowing Ar.

As a result of the growth process, six GaN/SiC samples were obtained inwhich the GaN thickness was in the range of 1 to 3 millimeters. Toremove the SiC substrates, the samples were first glued to metal holdersusing mounting wax (e.g., QuickStick.™ 135) at a temperature of 130° C.with the GaN layer facing the holder. The holders were placed on apolishing machine (e.g., SBT Model 920) and a thick portion of the SiCsubstrates were ground away using a 30 micron diamond suspension at 100rpm with a pressure of 0.1 to 3 kilograms per square centimeter. Thisprocess was continued for a period of between 8 and 24 hours. Afterremoval of between 200 and 250 microns of SiC, the samples were ungluedfrom the holders and cleaned in hot acetone for approximately 20minutes.

The residual SiC material was removed from each sample using a reactiveion etching (RIE) technique. Each sample was placed inside a quartzetching chamber on the stainless steel holder. The RIE was performedusing Si₃F/Ar for a period of between 5 and 12 hours, depending upon thethickness of the residual SiC. The etching rate of SiC in this processis about 10 microns per hour. After the RIE process was completed, thesamples were cleaned to remove possible surface contamination. As aresult of the above processes, freestanding GaN plates completely freeof any trace of SiC were obtained.

After completion of a conventional cleaning procedure, the GaN plateswere placed in the HVPE reactor. A GaN homoepitaxial growth was startedon the as-grown (0001) Ga surface of the GaN plates. The growthtemperature was approximately 1060° C. After a period of growth of 10minutes, the samples were cooled and unloaded from the reactor. The GaNlayer grown on the GaN plates was intended to cover defects existing inthe GaN plates. Accordingly, the samples at the completion of this stepwere comprised of 5.08 centimeter diameter (i.e., 2 inch diameter) GaNplates with approximately 10 microns of newly grown GaN. Note that forsome samples a GaN layer was grown not only on the (0001) Ga face of theGaN plates, but also on the (0001) N face of the plates. Peripheralhighly defective regions of the GaN plates were removed by grinding.

Three of the GaN plates from the previous process were loaded into thereactor in order to grow thick GaN boules. Gallium chloride and ammoniagas served as source materials for growth as previously disclosed. Inaddition, during the growth cycle the GaN boules were doped with siliconsupplied to the growth zone by S₂H₄ gas. Growth temperatures ranged from970° C. to 1020° C. and the growth run lasted for 48 hours. Three bouleswith thicknesses of 5 millimeters, 7 millimeters, and 9 millimeters,respectively, were grown.

The boules were sliced into GaN wafers. Prior to wafer preparation, someof the boules were ground into a cylindrical shape and peripheralpolycrystalline GaN regions, usually between 1 and 2 millimeters thick,were removed. Depending upon wafer thickness, which ranged from 150 to500 microns, between 7 and 21 wafers were obtained per boule. The waferswere then polished on either one side or both sides using an SBT Model920 polishing machine with a 15 micron diamond suspension at 100 rpmwith a pressure of between 0.5 and 3 kilograms per square centimeter for9 minutes per side. After cleaning all parts and the holder for 5 to 10minutes in water with soap, the polishing process was repeated with a 5micron diamond suspension for 10 minutes at the same pressure. Aftersubjecting the parts and the holder to another cleaning, the wafers werepolished using a new polishing cloth and a 0.1 micron diamond suspensionfor an hour at 100 rpm with a pressure of between 0.5 and 3 kilogramsper square centimeter.

After cleaning, the GaN wafers were characterized in terms of crystalstructure, electrical and optical properties. X-ray diffraction showedthat the wafers were single crystal GaN with a 2H polytype structure.The FWHM of the x-ray rocking curve measured in .omega.-scanninggeometry ranged from 60 to 360 arc seconds for different samples. Afterchemical etching, the etch pit density measured between 100 and 10,000per square centimeter, depending upon the sample. Wafers had n-typeconductivity with a concentration N_(d)−N_(a) of between 5 and 9×10¹⁸per cubic centimeter. The wafers were used as substrates for devicefabrication, particularly for GaN/AlGaN multi-layer device structuresgrown by the MOCVD process. Pn diodes were fabricated using a verticalcurrent flow geometry, which was possible due to the good electricalconductivity of the GaN substrates.

As will be understood by those familiar with the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. Accordingly, thedisclosures and descriptions herein are intended to be illustrative, butnot limiting, of the scope of the invention which is set forth in thefollowing claims.

1. A method of producing a GaN single crystal, the method comprising:providing a GaN seed in a furnace; providing a GaN powder in thefurnace; sublimating the GaN powder to grow a GaN crystal on the seed,the crystal having a volume exceeding 4 cubic centimeters, wherein an x,a y, and a z dimension of said GaN single crystal each exceed 1centimeter.
 2. A method according to claim 1 and comprising holding theGaN powder in an extended source boat.
 3. A method according to claim 2wherein the GaN powder in the boat is maintained at more than onetemperature.
 4. A method according to claim 3 wherein a portion of theGaN powder greater than 90% is maintained at a relatively lowtemperature.
 5. A method according to claim 4 wherein the lowtemperature is less than 100° C.